1. Field of the Invention
The invention relates to sheet-fed rotary printing presses wherein a plate cylinder cooperates with an impression cylinder. More particularly, the invention pertains to sheet-fed rotogravure presses, in which the impression cylinder is provided with grippers for holding individual printed sheets and which are cyclically swiveled into the impression cylinder in such a way that the grippers are totally within the cylinder contour. Specifically, the invention relates to a mechanism having gripper mounting brackets that swivel about a longitudinal axis lying at the leading edge of the cylinder surface.
2. Description of the Prior Art
U.S. Pat. No. 1,535,224 to C. W. Harrold issued Apr. 28, 1925 illustrates the well-known use of grippers, mounted on a rocking gripper shaft that is rocked by a cam follower on a pivot arm secured to the shaft and spring pressed against a cam surface fixed to the machine frame, for gripping individually fed sheets in a rotary printing press.
West German Patent Application No. 1,611,297 published Dec. 10, 1970 discloses a mechanism which rotates the point at which individually fed sheets are gripped about a longitudinal axis lying at the leading edge of the cylinder surface by means of a system of cranks and levers driven by a cam follower following a cam surface fixed to the machine frame.
West German Pat. No. 2,135,714 issued June 20, 1974, discloses a gripping mechanism that grips sheets that are individually fed to a rotary press followed by movement of the grippers radially inward below the surface of the cylinder driven by a cam follower following a cam surface fixed to the machine frame.
On rotogravure presses it is known that the cylinders used preferably have a narrow gap, particularly those with so-called wrap-around plates. This is firstly because large cylinder gaps on rotary presses lead to non-printable zones resulting in waste. Secondly, the ink on the plate cylinder must be removed by a doctor blade. But a large cylinder gap causes an accumulation of ink and an increased tendency for the blade to "fall into the gap" with resultant vibration and spattering of ink. Thus, with a plate cylinder for gravure printing the aim is always to provide a "closed" cylinder without a substantial cylinder gap.
An obstacle to the use of a closed plate cylinder in sheet-fed rotogravure presses is that the impression cylinder must have grippers to hold the individual printed sheets. It is desirable to have the backs of the grippers extend beyond the contour of the impression cylinder so that the grippers may easily receive the fed sheets. But the grippers must retract into the impression cylinder to permit the plate cylinder to cyclically roll against the impression cylinder at the gripper position.
The German Pat. No. 2,135,714 provides for cyclically retracting the grippers within the plate cylinder contour, but has the disadvantage that the gripped edge of the fed sheet is pulled against the leading edge of the cylinder at the risk of tearing the sheet.
The arrangement disclosed in German Patent Application No. 1,611,297 utilizes a gripper which retracts within the impression cylinder contour while also preventing binding and scraping problems by using a swiveling mechanism that swivels the grippers about a longitudinal axis lying at the leading edge of the impression cylinder surface. The gripper and gripper pad are inserted by a roller, which is moved by a control cam. This arrangement, however, has the disadvantage that play in the system leading to premature wear is unavoidable in the lever gear and the associated pivots. A further disadvantage is that the gripper pad is mounted with the gripper on an additional lever, which has a further fixed point of rotation. Another important disadvantage of the arrangement is that the gripper shaft is mounted at only two points and thus does not have the stability required for high quality, high speed printing.